1. Field of the Invention
The present invention relates generally to output buffers of the type employed to control the voltage on an output pad of a semiconductor device, and, more particularly, to a circuit for use with a self-bootstrap type n-channel output buffer that augments the self-bootstrapping capability of such buffer when the output pad transitions to a logic one state from an initial state with a non-zero voltage magnitude.
2. Description of the Related Art
A self-bootstrapping buffer, for example, of the type using a pullup n-channel field effect transistor, such as disclosed in my copending application entitled "Ultra Low Power Pumped N-Channel Output Buffer With Self-Bootstrap", employs a gate-to-source capacitance C.sub.gs of the pullup transistor to bootstrap an input data signal in order to provide the needed pumped gate drive signal. In this configuration, a drain terminal of the pullup transistor is tied to V.sub.cc, a source terminal is tied to the pad, and the gate terminal receives the input drive signal. In particular, a potential problem arises when the input data signal is driven high to turn-on the pullup transistor but the output node voltage is not at a logic zero. This situation may occur, for example, when the output of the buffer is tri-stated, and, the output node has been pulled to a logic one by some other device, or, perhaps, when the input data is changing very fast so that the output node does no more than "glitch" towards a logic zero state before being driven back to a logic one state.
When the output node is not near ground potential (i.e., zero volts), then the self-bootstrapping will either not occur, or, alternatively, will not happen to a sufficient degree. For example, if the output node (i.e., a first capacitor terminal of C.sub.gs) is at a logic one, then, when the input data signal transitions from a low to a high state, thus placing a logic high on a second capacitor terminal of C.sub.gs, there is no final voltage developed across C.sub.gs. Accordingly, once the pullup transistor conducts, the boost amount developed on the gate terminal of the pullup transistor, does not rise appreciably above the device operating voltage level V.sub.cc. Under these circumstances, the pullup transistor having no "pumped" gate drive signal on the gate terminal, would be incapable of generating the desired output voltage (V.sub.OH) (for example, V.sub.cc when V.sub.cc =3.3 volts), and current (I.sub.OH) levels for the logic high state.
Accordingly, there is a need to provide a circuit and method for use with a self-bootstrapping output buffer of the type suitable for driving an output pad of the semiconductor device, that reduces or eliminates one or more of the problems as described above, and, in particular, addresses the problem related to the impairment or failure of the self-bootstrapping mechanism which may occur when the output node is not at zero volts when the input data signal begins to change from a logic zero to a logic one.